You're allowed one pokemon as your lifelong partner, that means you can't evolve it if you choose a pokemon that's not in its final form.
The pokemon can't be a legendary of any sort, and the base stat can't be 600 or over, so it can't be Dragonite/Salamence etc., however their pre-evolutions are allowed.

I'm going to assume alternate formes are allowed as well, seeing as they aren't exactly "final". If that indeed is the case, I'd choose Rotom. It would have some amazing real-world applications, once you think about it. Imagine those perfect lawns, and all without you having to lift a finger.